Process of producing calcined borax



Patented Mar. 3, 1936 UNITED STATES PATENT OFFICE PROCESS OF PRODUCINGGALCINED ORAX Original application March 11, 1929, Serial No.

346,107. Divided and this application September 24, 1934, Serial No.745,250

7 Claims.

This invention relates to a process of producing calcined borax. Thisapplication is a division of my co-pending application, SerialNo.346,107, filed March 11, 1929.

The ordinary borax of commerce is the fully hydrated form(Na2B4O7.10H2O) This product in a fine granular form has an apparentdensity of 0.90 to 1.0 and contains but 52.9 per cent of Na2B40'7, theremaining 47.1 per cent of the product being water.

A considerable saving can be made in transportation charges if the fullyhydrated borax of commerce can be properly converted to a partiallydehydrated or calcined form. However, it is a well known characteristicof borax to intumesce or puff to a large volume when it is heated tobring about calcination. In the past, it has been considered impossibleto produce a calcined borax containing less than three molecules ofwater of crystallization (approximately 21% of water) without havingintumescence or putting occur. As a result of this behavior of borax tointumesce upon heating, the dehydrated or partially dehydrated productson the market today under the name of calcined borax are veryvoluminous, dusty materials usually in the form of a powder. By dustymaterial is meant borax in such finely divided state and of such lowdensity as to readily float on air like ordinary dust. Such a productmay have an apparent density as low as .05 of that of water. Thebulkiness of the product necessitates a considerable increase in thecost of containers, such as sacks, or barrels per ton of borax andincreased freight rate, thus tending to offset the advantage to begained by shipping a product containing less water.

Manufacturers of borax products have attempted to overcome thisdifiiculty by fusing the calcined borax at higher temperatures toproduce an essentially anhydrous dense product, known as borax glass.Besides the additional expense involved, considerable difliculty is metwith in this operation due to the bulkiness and porosity of the calcinedborax and its resistance to the transfer of heat. It is also difficultto construct satisfactory fusion furnaces due to the highly corrosivenature of the molten borax. Another disadvantage ofthe borax glass isits very slow rate of solution in water.

It is the principal object of this invention to produce a calcined boraxof a relatively dense nature without the necessity of fusing to a glass.It is another object of the present invention to produce a densecalcined borax having a relative: 1y rapid rate of solution in water.

I have discovered that the production of a calcined borax withoutintumescence may be accomplished by subjecting crystal borax to theaction of heated air or to other suitable gas in such a manner that thetemperature of the air with which the borax is in contact never becomeshigh enough to cause the borax to melt or go into solution, eitherwholly or in part, in the remaining water of crystallization containedtherein, provided that at the same time the humidity of the air incontact with the borax is kept low enough to cause the borax to besubjected to a dehydrating action. When borax is brought into contactwith air having a partial pressure of water vapor below that exerted bythe solid borax, so that moisture will continue to be transferred fromthe borax to the air and at the same time the air is kept atsufificiently low temperature that the borax will not melt, dehydrationmay be effected without intumescence or puffing of the borax. In theforegoing discussion of the action of borax in melting in its own watercrystallization, it should be understood that in some cases the actionmay be so severe as to cause the entire mass undergoing dehydration tomelt together into one liquid, but in the majority of cases wherepufling is experienced the crystals melt up individually, and generallyonly partially. Thus, while in some cases the dehydrating conditions maynot be such as to cause the whole body of borax to melt together, stillon account of the individual crystals of borax melting either entirelyor on their surface the water vapor becomes trapped and furtherdehydration proceeds with a pulling and enlargement of the crystals.

In accordance with the discovery of this invention, a process ofcalcining borax is provided in which the borax crystals or particlesretain essentially the same shape and size as those of the originallyhydrated material. Thus, by starting with a dustless granular borax, arelatively dense, dustless, granular calcined product is produced. Itappears clearly that intumescence or pufiing of borax upon heating, suchas has been experienced in the prior attempts to calcine borax, is dueto a state of fusion of the surface of the mass of borax undergoingdehydration, after which the further ebullition of water causes thepuffing action. By calcining the borax in accordance with the principlesof the present invention, always at a temperature below the meltingpoint of the borax or below the point that the borax goes into solutionin the remaining Water of crystallization, intumescence or puffing isentirely avoided.

The present invention, together with various additional objects andadvantages thereof will be best understood from a description of apreferred calcined borax and a preferred process of producing the same.For this purpose, there is hereafter described a process of calciningborax and the preferred calcined product.

The description is given in connection with the accompanying drawing,which is diagrammatic in character.

The temperature at which borax melts or dissolves in the water ofcrystallization present varies, depending upon the amount of water ofcrystallization present in the borax. In general, it is true that thehigher the percentage of water of crystallization in the borax, thelower the temperature at which the borax melts or dissolves in its waterof crystallization. It is further found thatthe absolute humidity of thewater vapor in the air employed to calcine borax must be lower, as thewater present in the borax being calcined becomes lower. This is trueeven though air of higher temperatures may be used in calcining boraxhaving a small percentage of water of crystallization.

There is hereafter described a process of calcining borax in which thetemperatures and humidity of air are controlled as the boraxprogressively loses its water of crystallization in order toproperlycalcine borax without pufling.

Granular borax (Na2B4O710I-I2O) is fed into chamber I, where it istreated with a stream of air or other drying gas at a temperature offrom 50 to C., having an absolute humidity of between 0.08 and 0.1pounds of water Vapor per pound of bonedry air. The air passes throughthe chamber at such a rate that both the inlet and outlet air remainswithin these specified limits. After about fifteen minutes, dehydrationhas proceeded to such an extent that the borax will not melt in itsremaining water of crystallization at the temperature employed in thenext chamber and the borax is then transferred to this chamber 2.corresponds approximately to Na2B40'78I-I20.

The air in chamber 2 is maintained between the temperature limits of 65and C. with an absolute humidity of between 0.06 and 0.08 pounds ofwater per pound of bone dry air. After remaining about fifteen minutesin this chamber, the borax is transferred to chamber 3. At this point inthe process, it has a'composition corresponding approximately to theformula Na2B40'z6H2O which has been found to be a suiiicient dehydrationto prevent melting at the temperature maintained in chamber 3.

The air in chamber 3 is maintained between the temperature limits of125' and 200 C. and the absolute humidity between 0.04 and 0.06 poundsof water per pound of bone dry air. The borax, upon leaving chamber 3has a composition corresponding approximately to the formulaN32B40'74H20, at which time it is sufficiently dehydrated-to preventmelting in chamber 4. In chamber 4, the air is maintained between thetemperature limits of to 260 C. and the absolute humidity of the watervapor between 0.02 and 0.04 pounds of water vapor per pound of bone dryair.

Borax from chamber No. 4 is fed into chamber 5 and has a compositioncorresponding approximately to the formula Na2B4O'z2H2O, and is sufiitheformula Its composition at this point ciently dehydrated to preventmelting in this chamber 5. In chamber 5, the air is maintained at atemperature between 225 and 325 C. and the absolute humidity of the airless than 0.02 pounds of water vapor per pound of bone dry air.

The borax leaving this chamber, after a period of about fifteen minutesin each of the chambers in turn, was found to consist essentially ofmonohydrate (Na2B4O1H2O). The particles of this monohydrate of boraxhave essentially the same shape as those of the original dekahydrate(Na2B4O'z10H2O), but they are more opaque due to the process ofdehydration.

The particles, however, are not sufiiciently friable to crumble to apowder with ordinary handling, such as the product receives in commerce.The product produced moreover is rapidly soluble in water and thusconsiderably superior to borax glass made by fusion. Instead of beingmore bulky than the original borax, there was an actual shrinkage inmaterial, as will be seen from the following data on the comparison ofweight of anhydrous borax (NazBiov) packed in While in the above tablethe product is shown as having an apparent density of 0.72, depending onsuch factors as rate of dehydration and percentage of water permitted inthe final product, the product may possess an apparent density ofbetween 0.5 and 0.9.

While in the process herein described, we have specified certain lowerlimits of humidity of the air going through the several chambers, theair may be passed through the chambers with humidities lower than thosegiven, but the lower limits specified are those ordinarily obtainablewithout a special process of drying the air.

It will be readily seen that as the air passes through the chambers, itshumidity automatically increases since it picks up water vapor from theborax. This causes the temperature of the air at the same time todecrease due to the sensible heat of the air being taken up to supplythe latent heat of evaporation of water of crystallization of the borax.Thus, after the air is passed through one of the latter chambers of theseries, its temperature and humidity are brought to those conditions ofthe air in the preceding chamber of the series. Thus, the air passingout of chamber 5 may be used as the air supply for chamber 4, and theair supply passing from chamber 4, used as the air supply for chamber 3,and so on in the series. Furthermore, in 'place of the series ofchambers, a single long chamber may be used, so arranged that the boraxand air are passed from end to end, counter-current to each other. Insuch an apparatus, the borax should be subjected to dehydratingconditions from about 1 to 3 hours. The time of calcination dependsmainly upon the size of the crystals, since the controlling factor isthe rate of diffusion of Water outwardly from the interior of thecrystals. The smaller the c'rystals'prosessed, the more rapidly they maybe dehydrated without overheating. In such a counter-current process,sufficient air must be provided in order to maintain the proper exitconditions in contact with the fresh incoming borax. As previouslystated, in connection with chamber I, the temperature of this exit airshould lie between 50 and 100 C. and the absolute humidity below 0.1pounds of water vapor per pound of bone dry air.

While the process of the present invention is suitable for dehydratingborax to any desired extent so as to secure a borax having less waterthan the dekahydrate, which contains about 43 per cent of water, theprocess of the present invention is of special value in dehydratingborax to reduce the percentage of Water in the borax to between thelimits of 8 per cent and 21 per cent, or to dehydrate the borax so thatit possesses less water of crystallization than three molecules andcontains at least one molecule of water.

Moreover, while the process of the present invention has been describedas applied to the calcination of borax dekahydrate (Na2B4O'110I-I2O)other hydrates of sodium tetraborate, such as pentahydrate(Na2B4O'1.5H2O) or mixtures of pentahydrate and dekahydrate aresuitable. Moreover, the process of the present invention may be appliedto various partially calcined products of either the dekahydrate orpentahydrate of borax, when the partial calcination of these productshas been carried on in other manners, provided that such partialcalcination has not excessively pufled the borax.

While the particular process herein described for the purpose ofillustrating the invention is well adapted to carry out the objects ofthis invention, it is to be understood that various changes andmodifications may be made without departing from the process of theinvention, and the invention includes all such changes and modificationsas come within the scope of the appended claims.

I claim:

1. A process of producing a calcined borax of relatively dense naturefrom relatively hydrated borax, which comprises subjecting the borax tothe action of inert gas of progressively increasing temperature anddecreasing absolute humidity while the calcination process proceeds, thesaid increasing temperature being held throughout the calcining processbelow the temperature which will cause the borax to undergo melting andpuffing, the humidity of the gas being at all times sufficiently lowthat at the temperature of the gas, the gas is capable of absorbingwater of crystallization from the borax undergoing calcination withoutincurring melting or pufiing of said borax, the action of the heatedgases on the borax being continued until a granular, dense, calcinedproduct containing less than 21% water is produced.

2. A process of producing calcined borax from a relatively hydratedborax, which comprises subj ecting the relatively hydrated borax to theaction of an inert gaseous phase of progressively increasing temperatureand progressively decreasing humidity as dehydration of the boraxproceeds, the said increasing temperature being held throughout thecalcining process below the temperature which will cause the borax toundergo melting and pulling, the humidity of the gas being maintainedthroughout the process suificiently low that the gas is capable ofabsorbing water of hydration from the borax without causing pufling ormelting of said borax, the process being continued until borax iscalcined to a composition equivalent substantially to the monohydrate ofsodium tetraborate.

3. A process of producing a calcined borax, which comprisessubjectingcrystal borax to the action of inert gas of progressivelyincreasing temperatures and progressively decreasing water content asdehydration proceeds, the said increasing temperature being heldthroughout the calcining process below the temperature which will causethe borax to undergo melting and pufilng, while the partial pressure ofwater vapor in the gas throughout the process is maintained sufficientlylow that it is capable of absorbing water of hydration from the boraxwithout causing melting or puffing of said borax, the action of the gasbeing continued until a granular product of dense nature containing lessthan 21% remaining water of crystallization is produced, the dehydratedgranular particles retaining substantially the same shape as thecrystals of the original borax.

4. A process of producing a calcined borax of relatively dense naturefrom a relatively hydrated borax, which comprises subjecting the boraxto the action of inert gas of progressively increasing temperature anddecreasing absolute humidity until a granular product correspondingsubstan tially to the monohydrate is produced, the said increasingtemperature being held between 50 and 100 C. while the borax compositioncorresponds to between between and 150 C. while the borax corresponds toto between 125 to 200 C. while the borax composition corresponds tobetween to between 175 C. and 260 C. while the borax compositioncorresponds to between N3.2B401.4H2O and Na2B40'L2H20,

and between 225 and 325 C. while the borax composition corresponds tobetween the humidity of the gas during said respective 0 periods beingrespectively not greater than 0.1, 0.08, 0.06, 0.04, and 0.02 pounds ofH20 per pound of dry air, whereby the temperature and humidity of thegas is held throughout the process sufllciently low that the boraxundergoes calcination without melting or pufiing of said borax.

5. A process of producing a calcined borax of relatively dense naturefrom a relatively hydrated borax, which comprises subjecting the boraxto the action of inert gas of progressively increasing temperature anddecreasing absolute humidity while the calcination process proceeds,until a granular product of dense nature containing less than 21%remaining water of crystallization is produce-d, the dehydrated granularparticles retaining substantially the same shape as the crystals of theoriginal borax, the said increasing temperature being held throughoutthe calcining process between the limits of 50 to C., 65'to 150 C., to200 C., 1'75 to 260 C., and 225 to 325 C., whenever the water content ofthe borax undergoing calcination during the process is respectivelybetween the limits of 10 to 8, 8 to 6, 6 to 4, 4 to 2, and 2 to 1molecules of water of crystallization, respectively, while maintainingthe humidity of the air during those respective periods .of dehydrationbelow respectively 0.1, 0.08, 0.06, 0.04 and 0.02 pounds of water perpound of dry air.

6. The method of producing calcined borax of relatively dense naturewhich comprises subjecting hydrated sodium tetraborate to the action ofheated air of progressively increasing temperature and progressivelydecreasing humidity, controlling the temperature of the heated air sothat it is between the range of 50-100 C. when the water content isbetween 8 and 10 molecules of water of crystallization, 65-150 C. whenbetween 8 and 6 molecules of water of crystallization, 125-200 C. whenbetween 6 and 4 molecules of water of crystallization, 175-260 0. whenbetween 4 and 2 molecules of water of crystallization, 225-325 C. whenbetween 2 and 1 molecules of water of crystallization and controllingthe humidity of the heated air during the dehydration treatment so thatit shall be less than 0.1 pound of water per pound of bone dry air forhydrates between 10 and 8 molecules of water of crystallization, lessthan 0.08 pound of water per pound of bone dry air when between 8 and 6molecules of water of crystallization, less than 0.06 pound of water perpound of bone dry air when between 6 and 4 molecules of water ofcrystallization, less than 0.04 pound of water per pound of bone dry airwhen between 4 and 2 molecules of water of crystallization and less than0.02 pound of water per pound of bone dry air when between 2 and 1molecules of water of crystallization, and continuing the dehydrationuntil less than 21% of water of crystallization remains.

7. A method of producing calcined borax of relatively dense nature,which comprises subjecting hydrated sodium tetraborate to thecountercurrent action of heated air of controlled humidity, controllingthe temperature of the heated air so that it is between 125 and 200 C.when between 6 and 4 molecules of water of crystallization; 175 and 2600. when between 4 and 2 molecules of water of crystallization; 225 and325 C. when between 2 and 1 molecules of water of crystallization; andcontrolling the humidity of the heated air during the dehydration sothat it is less than 0.06 pound of water per pound of bone dry air whenbetween 6 and 4 molecules of Water of crystallization; less than 0.04pound of bone dry air when between 4 and 2 molecules of water ofcrystallization; and less than 0.02

pound of bone dry air when between 2 and 1 molecules of water ofcrystallization.

WILLIAM H. ALLEN.

